In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is typically coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gear set.
Shifting from a currently established speed ratio to a new speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio. Each such shift includes a fill or preparation phase during which an apply chamber of the on-coming clutch is filled in preparation for torque transmission. Once filled, the on-coming clutch transmits torque in relation to the clutch pressure, and the shift can be completed using various control strategies.
In a clutch-to-clutch transmission, disengagement of the off-going clutch and engagement of the on-coming clutch is accomplished by a transmission controller transmitting pressure commands to alter the pressure and fluid volume in the respective apply chambers. The transmission controller must take into account such variables as the volume of fluid necessary to fill each clutch's respective apply chamber, fluid flow rates, fluid temperature, etc., in generating the pressure commands to ensure proper timing of the clutches. If an on-coming clutch gains torque capacity prior to disengagement of the off-going clutch, then transmission tie-up may occur. If an on-coming clutch gains torque capacity too long after the off-going clutch disengages, then engine flare will occur.
The prior art includes clutch-to-clutch transmissions that employ closed loop control of the off-going clutch during a shift event such that the off-going clutch maintains a predetermined slip threshold. As the on-coming clutch gains torque capacity, the speed of the input shaft drops. The transmission controller, as a result of the closed loop control of the off-going clutch, will compensate by reducing the pressure command for the off-going clutch as the on-coming clutch gains capacity until the off-going clutch torque capacity is zero.